Started work on clang object serialization. Experimental

serialization logic as well as driver code is now in
Driver/SerializationTest.cpp.  The status of this code is that it
should be used by no clients.

Added --test-pickling option to driver to run the serialization code.

Modified IdentifierInfo and IdentifierTable to have friend classes
that permit object serialization.  Such friendship may not be needed
in the final design.

llvm-svn: 43052